Both the original drives were Dell Certified Seagate Barracudas for enterprise, 250gb.

The new HDD is a Seagate Barracuda 500gb, not Dell-certified.

When I swap the failed drive with the new one, the SAS console shows that the drive status is "missing" -- and continues to show the primary drive from the original RAID-1 setup as the only one connected.

Does the swapped drive need to be dell-certified to work, or am I doing something wrong?